My team and I build about a dozen banner ads a week. We just do straight HTML/js, we haven't found a graphic banner tool worth the trouble. We rely on gsap timeline and Google fonts, neither of which add file weight because of the CDN. Most of our file weight is graphics unless we add a gsap members files and those only add a KB or two. Coupled with how awesome Greensock tutorials are and their response on this forum is we are getting by like bandits.
With that said it is my opinion that if you do as many banners as we do straight HTML is the way to go, if you only do one a month I would use Google Web Designer but I would also quit my job.
